1. Field of the Invention
This invention relates to vehicle seat belt retractors of the type which include a belt storage device, usually a spool, and a clamping device which engages the belt to prevent protraction thereof from the storage device. Such a clamping device is often called a "web locker".
2. Description of Prior Art
Examples of vehicle seat belt retractors which include web lockers can be found in patent specifications GB-A-2113978, GB-A-2126876 and GB-A-2177891. In all these specifications, the retractor has a belt storage spool and a web locker which comprises one or a pair of clamping wedges. The or each wedge has a planar front face for engagement with the belt and extending parallel to the plane thereof. A rear face of the or each wedge, inclined to the front face, is in sliding contact with an oppositely-inclined surface on the retractor frame. The wedge or wedges are moved towards the belt, and into clamping engagement therewith, when they are acted upon by an operating means of the retractor which produces a force directed substantially in the direction of protraction of the belt. The movement of the wedge(s) into engagement with the belt has a substantial component in the direction of belt protraction.
A web locker retractor of a different type is described in patent specification EP-A-0185367. Here a clamping block mounted on a swinging arm is brought into clamping engagement with the belt in order to arrest protraction of the belt from a storage spool. The surface of the block which contacts on face of the belt has a close-packed array of teeth, whilst a surface of the retractor frame with which the other face of the belt is pushed into contact has a low coefficient of friction and is, for example, a surface of a layer of polytetrafluoroethylene. In contrast to the retractors discussed above, the clamping block moves to its clamping position substantially perpendicularly to the plane of the belt; this movement has no significant component in the direction of belt protraction.
Patent specifications DE-A-3306434 and US-A-3504867 describe safety belt systems in which web-lockers having corrugated jaws are operable to divert a run of seat belt from a linear path into a zig-zag path, in which protraction of the belt is prevented by a capstan-like action of the jaws on the belt. In these web-lockers, a component of the tensile load on the belt acts against the web-locker clamping force and therefore tends to resist clamping. This increases the response time of the web-locker and requires the web-locker to be mounted in a structure which is very strong and rigid and correspondingly heavy.